190 



THE PROBLEM OP THE METALLIPEROUS VEINS. 



Gold percentage one tenth as much as silver. 



Tin percentage in the fourth or fifth decimal place — that is, in the 

 ten thonsandths or hundred thousandths of a per cent. 



These figures, inconceivably small as they are, convey some idea 

 of the rarity of these metals as constituents on the average of the 

 outer 6 or 8 miles of the earth's crust. But they are locally more 

 abundant in particular masses of eruptive rocks Avhich are associated 

 with ore deposits. 



In the following tabulation I have endeavored to luring together a 

 number of determinations which have been made in connection with 

 investigations of American mining districts. In a general way they 

 give a fair idea of the metallic contents of certain eruptive rocks from 

 which were taken samples as little as possible open to the suspicion 

 that they had been enriched by the same processes which had pro- 

 duced the neio:hborine: ore bodies. 



" Average of eight eruptives from Missouri, anal, hy J. D. Robertson. Report on Lead 

 and Zinc, Mo. Geol. Surv., II, 479. 



''Average of six different rocks, emln'acing eighteen assays; S. V. I^mmons, Monograph 

 XII, U. S. Geol. Surv., .591. 



" One rock, a quartz porphyry, not certain the rock was not enriched. J. I). Curtis, 

 U. S. Geol. Surv., Mono. VII, 136. 



'' S. F. Emmons, XVII Ann. Rep. U. S. Geol. Surv.. Part II, p. 471. The zinc was 

 determined in hut two samples. 



« S. F. Emmons, XVII Ann. Rep. U. S. Geol. Surv., Part II, p. 471. 



f Idem, p. 594. 



"A. Simundi in Tenth Census. XIII, 54. 



In order to come within the possible limits of profitable and suc- 

 cessful treatment the ores of the more important metals should have 

 at least the following percentages, but that we may grasp the rela- 

 tions correctly it must be appreciated that local conditions affect the 

 limits. Thus in a remote situation and with high charges for trans- 

 portation an ore may be outside profitable treatment, although it 

 may contain several times the percentages of those more favorably 

 situated. Iron ores in particular which are distant from centers of 

 population are valueless unless cheap transportation on a very large 

 scale can be developed, while gold in an almost inaccessible region, 

 like the Klondike, may yield a rich reward, even when in quantities 

 which, if expressed in percentages, are almost inappreciable. 



